There is a maximum grade and a minimum grade that you can achieve at these levels.
Higher---> A*-D
Foundation----> C-G
There is no longer an intermediate level, but it did used to be grades
B-F. Teachers normally only enter you in for the higher tier if they believe you could achieve at least a grade C. This is because you will get ungraded (U) if you achieve lower than a D, whereas there is a safety net with foundation paper as you could get lower than a D, however this would still be a fail at GCSE level.